Robotically controlled instruments are often used in minimally invasive surgical procedures. One common architecture for such surgical instruments includes an end effector or tool such as forceps, a scalpel, scissors, a wire loop, or a cauterizing tool mounted at the distal end of an extension, which is sometimes referred to herein as the main tube of the instrument. The distal tip of a robotically controlled instrument typically includes a wrist mechanism between the main tube and the end effector that allows for manipulating, positioning, or orienting the working surfaces of the end effector. During a surgical procedure, the end effector, the wrist mechanism, and the distal end of the main tube can be inserted through a small incision or a natural orifice of a patient and directed as needed to position the end effector at a work site within the body of the patient. Tendons, which can be cables or similar structures, extend through the main tube of the instrument and connect the end effector to a transmission and actuation mechanism, which is sometimes referred to herein as a backend mechanism. For robotic operation of the surgical instrument, the backend mechanism at the proximal end of the instrument is motor driven to pull on the tendons and thereby move or otherwise operate the wrist mechanism and end effector, and a computing system may be used to provide a user interface for a surgeon to precisely control the instrument.
Robotically controlled surgical instruments are being developed that have flexible main tubes that are able to bend as needed to follow a natural lumen such as a portion of the digestive tract of a patient or for insertion through a curved guide tube that provides an improved approach direction to the surgical site when compared to a straight approach. Whether inserted directly or through a guide, the main tubes of these flexible medical instruments will generally have several bends at locations that may vary during a procedure and may vary from one procedure to the next. At these bends, the tendons running through the instrument may rub against the inside wall of the main tube of the instrument and against each other, and friction generated due to these bends (sometimes referred to as capstan friction) can greatly increase the forces required to move the tendons to operate the wrist and end effector at the distal end of the main tube. Furthermore, these frictional forces tend to be higher at zero velocity than at low non-zero velocities, resulting in what is called stick-slip motion (sometimes referred to as stiction) in response to changes in tendon load. This stick-slip motion makes smooth robotic control of small motions of the instrument distal joints difficult to achieve. The large friction also makes construction of small-diameter flexible surgical instruments more difficult because mechanical structures must be designed to be robust enough to withstand the large forces. Accordingly, structures and methods for reducing the capstan friction encountered in flexible surgical instruments are desired.